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Not Applicable
1. Field of the Invention
The present invention relates to mine roof bolts, and more particularly relates to tensionable mine roof bolts constructed of multi-strand steel cable.
2. Description of Related Art Including Information Disclosed Under 37 C.F.R. xc2xa71.97 and 37 C.F.R. xc2xa71.98
In the art of mine tunnel roof support, there are two major categories of bolting systems wherein mine roof bolts are anchored in bore holes drilled in the mine tunnel roof, the bolts"" purpose being to reinforce and stabilize the unsupported rock formation above the mine tunnel. These two categories of mine roof bolting systems are: (1) tension-type systems, and (2) passive-type systems. In each system, it is common practice to, first, drill a hole through the mine tunnel ceiling into the rock formation above to a depth appropriate for the type of rock formation to be supported. A mine roof bolt and roof plate are then anchored in the bore hole to support the mine roof and maintain the rock formation in place.
In a common tension-type mine roof bolt system, an expansion shell type anchor is installed on the threaded end of the bolt. The bolt and expansion shell anchor are inserted up into the bore hole until the roof plate is against the mine roof. The bolt is then rotated to thread a tapered plug section of the expansion shell down toward the bolt head, in order to expand the jaws of the expansion shell against the interior wall of the bore hole to thereby hold the mine roof bolt in place within the bore hole, the mine roof bolt functioning to support and stabilize the rock formation above the mine tunnel.
In passive-type mine roof bolt systems, the passive-type bolt is not attached to an expansion shell or similar anchor at the free (upper) end of the bolt, but rather is retained in place within the rock formation by a rapid-curing resin adhesive material that is mixed in the bore hole as the bolt is rotated and positioned within the bore hole. In theory, the resin adhesive bonds the bolt to the rock formation along the total length of the bolt within the bore hole in the rock formation. It is also common practice to use resin adhesive with a tension-type mine roof bolt to retain the bolt within the mine roof bore hole, at least along the upper portion of the bolt. Again, when the rock formation shifts, certain types of tension system bolts can be retightened by rotating the bolt or nut.
In passive-type and some tension-type mine roof bolting systems, one or more resin cartridges are inserted into the bore hole prior to (ahead of) the mine roof bolt. Forcing the mine roof bolt into the bore hole while simultaneously rotating the bolt ruptures the resin cartridge(s) and mixes the resin components within the annulus between the bolt shaft and bore hole wall. Ideally, the resin adhesive mixture totally fills the annulus between the bolt shaft and bore hole wall at least along the upper portion of tension-type bolting systems, and along the total length of the bolt shaft and bore hole wall in passive-type systems. The resin mixture is forced into cracks and crevices in the bore hole wall and into the surrounding rock formation to adhere the bolt to the rock formation.
When extremely long mine roof bolts are necessary, it is common practice to attach two or more bolt shaft sections together by couplers to result in a xe2x80x9croof boltxe2x80x9d of sufficient length appropriate for the particular type of rock formation. These couplers between bolt sections, being of a larger diameter than the bolt shafts, prevent the mixed resin adhesive from flowing downwardly (resin return) within the bore hole annulus from the first (upper) bolt section to the lower section(s). Therefore, the effective anchoring of the bolt to the bore hole wall within the rock formation is, essentially, only along the length of the first (upper) bolt section wherein the resin adhesive totally fills the annulus between the bolt section and the bore hole wall.
To alleviate this problem, it has been common practice simply to drill a larger bore hole in the rock formation that will enable the resin adhesive to flow around the coupler(s) as the bolt is being inserted into and rotated within the bore hole to mix the resin. Although this does effect the desired result (resin return around the coupler(s) within the annulus between the bolt shaft and bore hole wall), it creates another problem that, depending on the type of rock formation, may be more dangerous than the problem that is corrected by a larger bore hole. Specifically, bonding of the resin adhesive material to hold the mine roof bolt in place within the bore hole is considerably weakened by virtue of the increased distance between the bolt shaft and bore hole wall, and the sheer volume of resin adhesive material necessary to totally fill the annulus. Additionally, by virtue of their specific makeups, mine roof rock formations that actually require long (fifteen feet or longer) mine roof bolts are more susceptible to movement and shifting within the rock formation, than are more solid rock formations that require only shorter mine roof bolts.
Another common problem with using mine roof bolt sections coupled together in such rock formations that require longer (coupled) mine roof bolts, this shifting of the rock formation (shear) causes the bolt couplers to fracture. When this happens, of course, the effective holding length of the mine roof bolt is instantly decreased. In many instances, there is no or very little resin adhesive material around the broken bolt shaft to help stabilize the rock formation. Therefore, in almost all instances, this shortened mine roof bolt is ineffective to safely prevent the mine roof rock formation from further shifting and potential collapse.
In response to these problems, so-called xe2x80x9ccable boltsxe2x80x9d have been devised for both tension-type systems and passive-type systems. A common design for a tension-type (tensionable) mine roof bolt has a bolt xe2x80x9cheadxe2x80x9d formed of a steel rod having an axial bore in one end and an externally threaded shaft at the other end. This xe2x80x9cheadxe2x80x9d is swaged down upon the cable to define a rigid threaded end (tension head) of the mine roof bolt for receiving a tensioning nut. The threaded section of the bolt head permits tensioning of the bolt within the rock formation above the mine tunnel after the resin mixture has set, and also permits subsequent re-tensioning of the mine roof bolt when the bolt loosens as a result of shifts in the rock formation.
In theory, the tensionable cable bolt alleviates the problems inherent in using solid mine roof bolt sections in a shifting rock formation above the mine tunnel. In practice, however, the above-described tensionable cable bolt has introduced a different problem. Specifically, because the cable is not solid, it tends to twist or torque as it is being initially tensioned upon installation and solidification of the resin material, and also as it is periodically re-tensioned when necessary. This twisting or torquing of the cable has two effects, depending on whether the tightening torque applied to the nut is in the same direction as the cable twist or in the opposite direction of the cable twist. If the nut-tightening torque is in the opposite direction of the cable twist, of course, the cable will unwind and separate. If the nut-tightening torque is in the same direction as the twist of the cable, the cable will torque and twist within the hole, and will actually draw up (shorten) in length between the anchor and the tension head and nut, due to the spiral orientation of the twisted cable. The effective xe2x80x9cshorteningxe2x80x9d of the free length of cable as it is being tensioned or re-tensioned, therefore, causes the cable to become taut prematurely, due to its artificially shortened length. Therefore, when the tensioning torque applied to the nut is released, the twisted cable is permitted to relax and untwist, thereby returning its effective length to the original pre-tensioned length. The effect of this is that the xe2x80x9ctensioningxe2x80x9d is lost, inasmuch as, due to the torquing and twisting of the cable within the bore hole, the xe2x80x9ctensioningxe2x80x9d turns out to be, in fact, false tensioning.
In addition, tensioning or re-tensioning tensionable cable bolts having the nut-tightening torque in the same direction as the cable twist also generates a xe2x80x9creverse torquexe2x80x9d or opposing torque stored as potential energy within the twisted and torqued span of cable. As the rock formation shifts, the bolt tends to untorque or untwist within the bore hole, and therefore increase its effective length from the anchor to the tensioning nut. Of course, this increase in effective length of the bolt relaxes tension in the xe2x80x9cfalsexe2x80x9d tensioned bolt to zero, rendering the bolt ineffective to maintain the rock formation above the mine tunnel in the compression necessary to maintain the integrity of the rock formation.
It is therefore an object of the present invention to provide a tensionable mine roof cable bolt that does not twist within the mine roof bore hole as the bolt is being tensioned or re-tensioned.
It is another object of the present invention to provide a tensionable mine roof cable bolt that does not change in length as it is being tensioned or re-tensioned within the mine roof bore hole.
It is a further object of the present invention to provide a tensionable mine roof cable bolt that does not store potential energy in the form of coil spring torque that can be released to loosen the bolt within the bore hole, when the rock formation above the mine tunnel roof shifts.
The tensionable mine roof bolt of the present invention is constructed of a length of pre-tensioned, multi-strand steel cable, commonly formed of six individual pre-tensioned steel strands spirally wrapped around a seventh steel strand. The head end of the bolt is formed of an outer sleeve that is externally threaded at least part of the way from the end, in order to accept a conventional tension nut. The cable bolt shaft includes an enlarged section that is slightly larger than the internal diameter of the bolt head outer sleeve so that the cable enlarged section slightly interferes with the bolt head outer sleeve as the enlarged section is pressed into the outer sleeve to form the cable bolt. The cable enlarged section is formed by the addition of a spacer sleeve around the cable center strand (king wire) and between the cable center strand and peripheral strands, or by the addition of a cable sleeve around the cable at the appropriate location. Either design will result in the cable enlarged section""s having a slightly greater outside diameter than the bolt head outer sleeve internal diameter bore, to result in the interference as the cable enlarged section is pressed into the bolt head outer sleeve.
In a preferred embodiment, the steel outer sleeve is swaged onto the cable to define a rigid threaded tension head for receiving the tension nut in a manner to result in the forming of a plurality of aligned wings formed on the tension head. The maximum external diameter of these wings is slightly greater than the internal diameter of the mine roof bore hole, so that the cable bolt tension head must be driven into the rock formation at the mine tunnel roof. The aligned wings function to prevent the bolt tension head from rotating within the bore hole in order to prevent twisting and torquing of the cable section of the bolt within the bore hole between the resined-in bolt anchor and tension head. Because the bolt tension head does not torque or twist relative to the anchor, the above-listed problems inherent in present-day tensionable cable bolts are eliminated.